688 



ECOLOGY 



suberized. Branch veins and veinlets usually are much more numerous 

 in xerophytic leaves than in the leaves of shade plants and hydrophytes. 



Parasitism and vascular development. When Orobanche fasciculata grows 

 parasitically on an Artemisia root (fig. 1083), the latter often is stimulated to unusual 

 development, the hadrome in particular being subject to extensive enlargement. In 

 the haustoria of Melampyrum, tracheids develop only after attachment to a host 

 plant. 1 Leaves infested by the parasitic fungus, Peronospora, sometimes develop 

 entirely new bundle tracts, certain primordia that commonly grow into mesophyll 

 developing instead into vascular tissue. In insect galls of Vitis (fig. 823) there is a 

 vast increase in hadrome, there being about the larval chamber a festoon of this 



tissue developed from the cortex. Some- 

 times in vascular tracts infested by para- 

 sites, parenchyma cells adjoining the 

 enlarged vessels become hypertrophied, 

 bulging out into the vessels as tyloses 



(P- 695)- 



Miscellaneous reactions of vascular 

 tissue. A potato tuber usually decays 

 after giving rise to new tubers or rhi- 

 zomes, which withdraw the food it had 

 accumulated. But if a tuber is planted 

 at the ground level in such a way that 

 sprouts developing in the air are con- 

 nected with the developing roots only 

 through the old tuber (fig. 1046), the 

 latter not only lives another season, but 

 many of its mature cells become once 

 more meristematic. Among such re- 

 juvenating tissues conductive elements 

 play the most important part, many 

 parenchymatic cells growing into tra- 

 cheids and becoming of importance in conducting water and salts to the growing shoots. 

 Similarly, when a leaf of Torenia is placed upright in the soil, it gives rise to a shoot; 

 the dorsiventral petiole develops into a radially symmetrical organ, and new vascular 

 bundles develop from parenchyma, forming a vascular cylinder comparable to that 

 of the stem. In many similar instances, where there is an increased flow of sub- 

 stances through the parenchyma, some of the cells in the latter may become trans- 

 formed into tracheids ; for example, when vascular bundles are severed, tracheids 

 and even tracheae may develop from parenchyma cells, in some cases forming 

 connecting " bridges " of conductive tissue between the severed bundles and neigh- 

 boring intact bundles. When scale primordia grow into leaves in air and light, there 



1 In the stem of Cuscuta and in parasites generally, the hadrome but not the leptome 

 is less developed than in green plants of similar size. Occasionally, also, parasitism 

 checks the growth of the organs infested by the parasite, the conductive tissues as well 

 as the others having a reduced development. 



FIG. 1018. A cross section of a leaf 

 segment of the hornwort (Ceratophyllum 

 demersum), showing an extremely simple 

 conductive bundle (v),made up of small un- 

 diff erentiated cells ; note the capacious air 

 chambers (a) and the mesophyll (), com- 

 posed of nearly uniform cells which con- 

 tain scattered chloroplasts (c), while the 

 epidermis (e) contains densely packed 

 chloroplasts; highly magnified. 



